Template : class specialization - c++

Question

I'm new in the C++ world.
Sorry for my nooby question.
I have a class
template <typename T>
class Foo
{
T t_;
void say_hello()
{ std::cout << "Ciao";}
// work with T ...
};
I want to specialize this template class for 3 types.
If type is (A or B or C), Then use this class
template<>
class Foo<A or B or C>
{
void say_hello()
{ std::cout << "Hello";}
};
What's the best way to do this?
Thank you for your help.

Answer 1

A possible solution uses SFINAE
template <typename T, typename = void>
class Foo
{
T t_;
void say_hello()
{ std::cout << "Ciao";}
// work with T ...
};
template <typename T>
class Foo<T, std::enable_if_t<std::is_same_v<T, A>,
|| std::is_same_v<T, B>,
|| std::is_same_v<T, C>>
{
void say_hello()
{ std::cout << "Hello";}
};
If you don't use T inside the Foo specialization (as in your example) you can also use a sort of self-inheritance
template <typename T>
class Foo
{
T t_;
void say_hello()
{ std::cout << "Ciao";}
// work with T ...
};
template <>
class Foo<A>
{
void say_hello()
{ std::cout << "Hello";}
};
template <>
class Foo<B> : public Foo<A>
{ };
template <>
class Foo<C> : public Foo<A>
{ };
Off Topic: if you want to use say_hello() outside the class, is better if you make it public (or if you declare Foo as a struct).

Answer 2

There are several possibilities, for example:
Specialization of the method only:
template<>
void Foo<A>::say_hello() { std::cout << "Hello"; }
template<>
void Foo<B>::say_hello() { std::cout << "Hello"; }
template<>
void Foo<C>::say_hello() { std::cout << "Hello"; }
or, in C++17, you might do:
template <typename T>
class Foo
{
T t_;
void say_hello()
{
if constexpr(std::is_same_v<T, A> || std::is_same_v<T, B> || std::is_same_v<T, C>) {
std::cout << "Hello";
} else {
std::cout << "Ciao";
}
}
// work with T ...
};
Whereas regular if works in that example, it would fail if you call code specific to A, B, C.
if constexpr won't have that issue.

Answer 3

A variant of the SFINAE solution that is a bit more concise for more classes.
template<class T, class... Ts>
struct is_one_of;
template<class T, class Ts>
struct is_one_of<T, T, Ts...> : std::true_type {}; //maybe add std::decay_t
template<class T, class S, class Ts>
struct is_one_of<T, S, Ts...> : is_one_of<T, Ts...> {};
template<class T>
struct is_one_of<T> : std::false_type{};
template<class T, class... Ts>
constexpr bool is_one_of_v = is_one_of<T, Ts...>::value;
template <typename T, typename = void>
class Foo
{
T t_;
void say_hello()
{ std::cout << "Ciao";}
// work with T ...
};
template <typename T>
class Foo<T, std::enable_if_t<is_one_of_v<T, A, B, C>
{
void say_hello()
{ std::cout << "Hello";}
};